286 SENSE OF SIGHT. 



street, or the trees of an avenue, that are nearest to us, or in the fore- 

 ground, form the largest images on the retina, and there is a gradual 

 diminution, so that, if we could imagine lines to be drawn along the 

 tops and bottoms of the objects, and to be sufficiently prolonged, they 

 would appear to meet in a point, as in Fig. 129. 



The art which traces objects, with their various degrees of apparent 

 diminution on account of distance, and of foreshortening on account 

 of obliquity of position, is called perspective. 



2. The intensity of light, shade, and colour. It has been shown, 

 that the intensity of light diminishes rapidly, according to the distance 

 of the body from which it emanates; so that it is only one-fourth as 

 powerful when doubly distant, one-sixteenth when quadruply distant, 

 and so on. This fact is early recognised ; and the mind avails itself 

 of it to judge, with much accuracy, of relative distances. It is, how- 

 ever, a pregnant source of optical illusions. In a bright sunshine, 

 mountains appear much nearer to us than when seen through the haze 

 of our Indian summer. 1 In a row of lamps along a street, if one be 

 more luminous than the rest, it seems to be the nearest ; and, in the 

 night, we incur the strangest errors in judging of the distance of a 

 luminous body. The sky appears nearer to the earth directly above, 

 than it does towards the horizon ; because the light from above having 

 to pass only through the atmosphere is but slightly obstructed, whilst 

 a portion only of that, which has to pass through the dense hetero- 

 geneous air, near the surface of the earth, arrives at the eye. The 

 upper part of the sky being, therefore, more luminous seems nearer ; 

 and, in the same manner, we explain, in part, why the sun and moon 

 appear larger at rising and setting. 



The shade of bodies keeps pace with their intensity of light ; and 

 accordingly, the shadows of objects near us, are strongly defined ; 

 whilst in the distance they become confused, and the light altogether 

 so faint, that the eye at last sees an extent of distant blue mountain 

 or plain, "appearing bluish," says Dr. Arnott, 2 "because the trans- 

 parent air, through which the light must pass, has a blue tinge, and 

 because the quantity of light arriving through the great extent of air 

 is insufficient to exhibit the detail." "The ridge called Blue Mount- 

 ains," he adds, "in Australia, and another of the same name in 

 America, and many others elsewhere, are not really blue, for they 

 possess all the diversity of scenery, which the finest climates can give ; 

 but to the discoverer's eye, bent on them from a distance, they all at 

 first appeared blue, and they have ever since retained the name." As 

 regards the Blue Ridge of America, Dr. Arnott labours under misap- 

 prehension. Within a very few miles from the whole of this extensive 

 chain, as well as from a distance, the blue tinge is perceptible, especially 



1 A delightful season, in the southern and western parts of North America more espe- 

 cially, generally occurring in October or November; and having nothing similar to it. so far 

 as we are aware, in any other part of the globe. It is dependent upon some meteorological 

 condition of the atmosphere, and occurs only when the wind is southerly, or from the warmer 

 regions; disappearing immediately as soon as it veers to the north. By some, this pheno- 

 menon has been supposed to be caused by the large fires in the western prairies; but the 

 warmth that attends the haze cannot be explained on this hypothesis, independently of other 

 sufficient objections to it. a Op. cit., p. 401. 



